invalid subscriber email address causes newsletter to get stuck forever

Hi,
I've recently upgraded from D5 to a Drupal 6 platform, with Simplenews subscriptions included. One of the subscriber email addresses transferred across was malformed and unsendable. In D5 Simplenews, this generated an error and was then ignored. In D6, this generates an error and an attempted resend on the next cron run. The newsletter then attempts and fails to resend on every future cron run. I've removed the subscription that causes the error, but the newsletter is still attempting to resend regardless. There appears to be no option to stop this behaviour short of editing tables. Would appreciate advice on fixing the tables causing this issue.